1. Field Of The Invention
The present patent invention relates generally to semiconductor technology. More specifically, the present invention relates to one-time user-programmable antifuses which have at least one metal electrode, and to methods for programming such antifuses which reduce the read-disturb phenomenon observed is such antifuses.
2. The Prior Art
User programmable antifuse devices have been used in programmable logic applications, for example field programmable gate arrays. An antifuse typically comprises a layer of antifuse material sandwiched in between two conductive electrodes. The antifuse device is initially an essentially open circuited device in its unprogrammed state and can be irreversibly converted into an essentially short circuited device by the application of a programming voltage across the two electrodes to disrupt the antifuse material and create a low resistance current path between the two electrodes.
One type of antifuses utilizes an N+doped substrate region as a lower electrode and an N+arsenic-doped polysilicon line as a second electrode. An oxide-nitride-oxide (ONO) layer sandwich is employed as the antifuse material. Such a device is described in U.S. Pat. No. 4,823,181 and U.S. Pat. No. 4,881,114. Antifuse programming methods for antifuses including at least one arsenic-doped electrode range from applying continuous DC programming potentials to applying pulsed DC programming potentials followed by AC "soaking" as described in U.S. Pat. No. 5,126,282. The method described and claimed in U.S. Pat. No. 5,126,282 employs the step of initially applying a DC programming potential difference such that the positive potential thereof is applied to the one of the antifuse electrodes containing the highest concentration of arsenic. U.S. Pat. No. 5,126,282 also teaches "soaking" already-programmed arsenic-electrode-containing antifuses with AC "soak" pulses to lower final antifuse resistance. The above-identified patent teaches the criticality of the polarity of the starting and finishing pulses in relation to the arsenic-containing electrode.
Another type of antifuse comprises an antifuse material disposed between a layer of polysilicon and a metal layer or between a pair of metal electrodes which may comprise different metal layers in a multilayer metal semiconductor fabrication process. The latter type of antifuse is referred to as a metal-to-metal antifuse. Such antifuses usually employ a layer of amorphous silicon as the antifuse material, although they may also employ other antifuse materials, such as oxide, nitride, oxide-nitride, nitride-oxide, oxide-nitride-oxide, nitride,-oxide-nitride layers, or combinations of amorphous silicon with thin dielectric materials mentioned above. The metal-to-metal antifuse offers the advantage of low antifuse resistance (on resistance) after programming. The low on resistance of the amorphous silicon metal-to-metal antifuse results from the substitution of metal as the conductive filament element in place of the polysilicon conductive filament of the first type of antifuse.
When a plurality of voltage pulses are applied to an antifuse to program it, the electric field is chosen to be above the breakdown voltage of the antifuse. The antifuse can change its state from a non-conducting (off) state, where its resistance is in the range of 1 Gohm, to a conducting (on) state where its resistance is less than 1 Mohm and typically in the range of tens of ohms to thousands of ohms. A conductive filament is formed between two electrodes.
It is well known in the literature that amorphous silicon antifuses can exhibit read-disturb (switching) behavior, namely the undesired changing of the antifuse from its programmed state back to its unprogrammed state. It has been observed that metal-to-metal amorphous silicon antifuses used in field programmable gate arrays exhibits this read-disturb problem. During operation, as current is passed through the programmed antifuse, its on resistance will be "disturbed" such that it will either increase until the resistance is such that the circuit functionality is affected, or will eventually result in an open-circuit condition. The read disturb problems of metal-to-metal antifuses and antifuses employing at least one metal electrode are generally not observed in the diffusion/ONO/polysilicon type antifuse. The present inventors have observed that presently-available data suggests that the metal/dielectric/metal antifuses, as well as metal/amorphous silicon/metal antifuses, exhibit a similar read-disturb problem.
The present inventors have observed that when a DC current of either polarity having a magnitude equal to 50% or more of the original programming current level is applied across the metal-amorphous silicon-metal antifuse, the antifuse will revert to its off state through the destruction of the conductive filament. The present inventors have observed that when a DC current of either polarity having a magnitude equal to 100% or more of the original programming current level is applied across the metal-dielectric-metal antifuse, the antifuse will revert to its off state through the destruction of the conductive filament. While the on resistance of a positively programmed antifuse can be disturbed by a positive DC voltage stress, the disturb probability is much higher for the reverse DC stressed antifuse. In addition, when the operation temperature is raised, the probability of read-disturb increases too. This problem severely limits the operating conditions of the metal-to-metal antifuse and thus restricts design flexibility.
There are several ways to reduce this problem. One programming method which helps to solve this problem comprises increasing the programming current or programming voltage. The result is that the antifuse operating current is at a level which is much less than the programming current. With high programming current or programming voltage, more power is generated during formation of the conductive filament, resulting in a filament having a larger diameter. Such a filament will have higher electromigration resistance and is less likely to be disturbed. In addition, since the operating current will be lower in comparison to the programming current, it adds to the electromigration immunity by providing lower current density through the programmed antifuse, resulting in no phase change or major material transport inside the conductive filament.
However, this approach has built-in penalties. Larger transistors are required to provide the higher programming current or programming voltage. This impacts the die size. In addition, lower operating current to prevent read-disturb reduces the speed of the product. Neither of these alternatives are optimal or desirable.
Another approach to minimizing read-disturb is to reduce the thickness of the antifuse material disposed between the antifuse electrodes. By reducing the thickness of this layer, a larger conductive filament can be created with the same programming voltage. By supplying the same power (I*V) to create a conductive filament, an antifuse with larger thickness will have a relatively smaller conductive filament diameter. During operation, such an antifuse will have higher operation current density and is thus more likely to be disturbed due to electromigration of material from the conductive filament. However, tradeoffs are required when providing a thinner antifuse material layer. Assuming use of the same composition of the antifuse material layer, a thinner antifuse material layer will result in lower breakdown voltage, higher leakage current, and increased capacitance. Its use may thus not be desirable since it impacts functionality and reliability.
One way to overcome this problem is to change the composition of the antifuse material. For example, an amorphous silicon antifuse layer may be replaced with a low-temperature dielectric. Dielectric materials, such as oxide, nitride, or combinations of oxide and nitride have lower leakage current and higher breakdown voltage. Therefore, to maintain the same breakdown voltage requirements, the thickness of the antifuse dielectric has to be reduced. However, reducing the thickness of the antifuse material layer results in an increase in the capacitance of the antifuse in its unprogrammed state. This increased capacitance has a negative impact on the product speed.
Because of the aforementioned drawbacks, the above-recited methods are not optimal solutions to the read disturb problem.
It is an object of the present invention to reduce the read disturb phenomenon observed in antifuses having at least one metal electrode in a manner which does not require either reducing antifuse thickness, or increasing the programming current.